Glass ceramic bodies, and method by boric oxide treatment

ABSTRACT

A method of making a strong, high expansion ceramic body resulting from thermal crystallization of an alumino-silicate glass body wherein the body is contacted with boric oxide vapors at a temperature above 1400* F. for at least 10 minutes until the boric oxide vapors have reacted with the surface layer of the body to form an integral surface layer of borosilicate glass on the body. Upon cooling, a compressive stress forms in the vitreous layer. The lineal coefficient of thermal expansion of the ceramic body is at least 50 X 10 7/*C over the range 0* to 300* C.

United States Patent Lynch 14 1 Apr. 4, 1972 GLASS CERAMIC BODIES, AND[56] References Cited METHOD BY BORIC OXIDE UNITED STATES PATENTSTREATMENT 3,384,508 5/1968 Bopp et al ..65/33 X [72] Inventor: CharlesS. Lynch, Toledo, Ohio 3,493,355 2/1970 Wu ..65/30 [73] Ass1gnee:Owens-Illinois, Inc. Primary Examiner Frank w. g [22] Filed: Jan. 13,1970 Attorney-W. A. Schaich and Charles S. Lynch [21] App]. N0.: 1,990[57] ABSTRACT Relaied Application Dam A method of making a strong, highexpansion ceramic body [63] Continuation of 583 410 Sept 30 1966resulting from thermal crystallization of an alumina-silicate abandonedglass body wherein the body is contacted with boric oxide vapors at atemperature above 1400 F. for at least 10 minutes 52 U.S. c1 ..65/3065/33 65/60 until the hric Xide have reacted with the Surface 65/3,1fi/124 layer of the body to form an integral surface layer of borosils1 1m. 01. ..C03b 29/00, C03c 15/00 icate glass on the body- P 01mg, achmpmssi"e stress 58 Field of Search ..65/30, 33, 60, 3; 117/124 formsin the vitreous layer- The lineal weh'lciem of thermal expansion of theceramic body is at least 50 X 10"/C over the range 0 to 300 C.

4 Claims, No Drawings GLASS CERAMIC BODIES, AND METHOD BY BORIC OXIDETREATMENT This application is a continuation of applicants copendingapplication Ser. No. 583,410, filed Sept. 30, 1966 now abandoned.

The present invention relates to glass-ceramic materials and formedbodies thereof. More particularly, the present invention relates toimproved glass-ceramics, and to techniques and methods for making same.

Glass-ceramics are frequently referred to in the art as semicrystallineceramic materials. These materials are made by in situ thermalcrystallization of a glass body to produce a multitude of fine crystalsdispersed throughout the body in the glassy matrix remaining after suchin situ crystallization.

Methods of making glass-ceramic materials are well known in the art andare described herein only in the interest of completeness in disclosure.Generally speaking, a crystallizable glass composition is melted andthereafter formed into the desired shape by conventional means such aspress molding, casting, blow molding, tube drawing or the like. Thesesolid glass, formed articles are then cooled down to about the glassannealing point or lower, and are heated in an initial low temperatureheat treatment range to form many nuclei or crystallites, and arethereafter heated, usually in a higher temperature range, to completethe crystallization to the desired degree. The optimum heat treatmentschedule depends, as will be understood, on the particular glasscompositionand its tendency to form nuclei, and the rate of formation ofnuclei and the rate of crystallization or crystal growth. Therefore, itis not possible to specify a heat treatment schedule that will be commonto all crystallizable glass compositions. In fact, it is possible tocarry out the entire nucleation and crystallization heat treatmentisothermally by employing a relatively long heat treatment time at arelatively low temperature suitable for nuclei formation, such as in therange from 100 F. below to 200 F. above the annealing point temperatureof the glass.

In actual practice, it has been found that the entire crystallizationprocess, including nucleation, can be accomplished on one lehr beltcontinuously advancing the article through successive temperature zonesto effect nucleation and crystallizatron.

While prior art glass-ceramic bodies have many desirable properties andare often suitable for use in certain types of tableware such as plates,cups and tea pots, as well as for various types of laboratory andspecialty applications, it would be desirable for many applications toincrease the basic of the strength of the semi-crystalline material.

Not all of the variables controlling the strength of glassceramics areunderstood, but there are two basic factors that must be considered inany strengthening process. It is well known that glass-ceramic materialsgenerally fail in tension, and secondly that these stress failuresoriginate at the surface of the body. Governed by these considerations,methods have been developed in the art to increase the strength of theseglass-ceramic materials by inducing a surface compressive stress. Thesecompressive stresses have heretofore been developed, for instance, byoverlaying a ceramic coating such as a slip of an enamel or glazeformulation, followed by a heat treatment to eliminate the slip vehicleand cause the glaze to fuse and mature, thus coating the glass-ceramicsubstrate. The glaze or enamel formulation chosen has a coefficient ofexpansion lower than the substrate, and thus on cooling compressivestress is developed in the surface layer. This process requires carefulformulation, preparation, and application of the glaze or enamelmaterial to assure the proper ratio of coefficients of expansion betweenthe glaze and the glass-ceramic body to be strengthened. It is wellknown in the art that an improper coefficient of expansion ratio willcause an unattractive appearance due to crazing or stress cracking, withthe concomitant result that there is no significant improvement instrength of the finished composite.

A substantial contribution to the art would therefore be realized if amethod were developed to increase the strength of glass-ceramics whileat the same time avoiding the shortcomings enumerated above.

It is therefore an object of the present invention to provide a methodof increasing the strength of glass-ceramic materials which is readilyincorporated into conventional glass-ceramic manufacturing operations.

It is a further object of the present invention to provide a novelmethod of producing a glass-ceramic whereby the final body bears asurface compressive layer of vitreous character, said vitreouscompressive layer being integrally joined to the semi-crystallineceramic substrate.

Another object is to provide a method whereby a surface compressivelayer can be developed on a glass-ceramic body by an economical process.

Another object is to provide a method of strengthening semi-crystallineceramic articles by the in situ formation of a substantially vitreousborosilicate surface layer.

A further object of this invention is to provide a method forstrengthening semi-crystalline alkali alumino-silicate materials throughthe formation of a vitreous, compressive surface layer. v

Another object is to provide a novel glass-ceramic body having anintegral vitreous surface layer under compressive stress.

Other objects, as well as aspects and advantages, of the presentinvention will become apparent from the following detailed description.

In attaining the objects of this invention, one feature resides inexposing an aluminosilicate glass-ceramic body or a vitreousaluminosilicate thermally crystallizable body, to vaporous boric oxide,whereby the boric oxide under the conditions of treatment forms asubstantially vitreous borosilicate compressive layer by reaction with asurface layer of said body.

It is especially efficacious to practice the present invention byexposing the formed glass body prior to crystallization to an atmosphererich in B 0 The glass body is heated in the presence of the B 0 or B 0pyrolyzable compound to a temperature sufficient to produce vaporous B 0which contacts the surface of the body and reacts therewith to form avitreous borosilicate layer thereon and integral therewith. The glassbody is maintained at a temperature for a time sufficient to produce asemi-crystalline ceramic body by in situ thermal crystallization whilein contact with the B 0 as hereinafter described. The semi-crystallinebody formed by the aforesaid treatment has an essentially vitreousborosilicate surface layer, resulting from reaction of the B 0 with thesurface layer of the silicate body.

In the foregoing embodiment of the invention a particular advantage isthat no extra heat treatment step is required, since heat treatment isnecessary for the crystallization of the glass to form theglass-ceramic, regardless of whether the 8 .0 vapors be present or not.On the other hand, the glass-ceramic can be previously made by in situthermal crystallization of the solid glass object or body, andthereafter contacted while at a temperature of at least l,400 F. for atleast 10 minutes with boric oxide vapors, thus reacting B 0 with surfaceportions of the glass-ceramic to form the vitreous layer of borosilicateglass integral with the surface of the glass-ceramic. While an extraheat treatment step may be necessary, one may wish to effect the boricoxide treatment in a separate kiln or furnace, thus avoidingcontamination of the main kiln with boric oxide.

It has been discovered that this process is particularly effective instrengthening an aluminosilicate glass-ceramic material that has acomparatively high coefficient of expansion. By this is meant an averagelineal coefiicient of thermal expansion at least 50 X l0 /C. over therange zero to 300 C., and preferably at least 70 X l0' C. over the samerange.

While it is to be understood that this invention can be practiced inconjunction with any silicate glass-ceramic possessing thermal expansioncharacteristics as described above, I have discovered that this methodcan be utilized especially effectively with respect to he system R 'O-AlO -SiO wherein R 0 represents the alkali oxides of Na O or Na O plus K0. These components plus a nucleant usually make up at least by weightof the total starting material glass or the final glassceramiccomposition. In addition, the composition can also contain compatiblemetal oxide or halide modifying agents, for instance, MgO, CdO, ZnO,C00, CaO, BaO, NaF, CaF LiF, MnO, FeO, NiO, B KF, PhD and P 0 It isusually necessary that there be at least one nucleating agent present.Nucleating agents such as C50 ZrO TiO and SnO are commonly used in totalamounts ranging from about 0.1 percent to about 12 percent by weight.

A now preferred class of such compositions of the base glass and theglass-ceramic formed therefrom by thermal in situ crystallizationcontain as the sole essential constituents the following components inthe following weight percentage ranges in the total composition:

Percent by Weight where the listed components comprise at least 85percent by weight of the glass or the glass-ceramic. The foregoingcompositions in the glass-ceramic or crystallized state have averagelineal thermal expansion coefficients of well over 50 X l0*/ C. over therange zero to 300C. I have found that such glass-ceramics, which containnepheline as the major crystalline species, can be effectively utilizedin the present process to form products of this invention.

Preferably, the glass-ceramics and glass precursors discussed in thelast paragraph (1) contain SiO in such an amount that its weight ratiorelative to the combined weight of (Na,O+b-20) is in the range from 2.1to 3, and (2) contain alumina in such an amount that the combined moleratio of (Na O+KO) to moles of A1 0 is at least 1.02, and (3) contain6-12 weight percent TiO as the nucleation agent selected.

Accordingly a glass of the foregoing description is melted and isthereafter formed by conventional means such as press molding, casting,blow molding, tube drawing, or the like.

The formed glass object, which has been cooled down to about itsannealing point or lower, is first heated in an initial low temperatureheat treatment range to form many nuclei or crystallites, and isthereafter heated usually in a higher temperature range, to complete thecrystallization to the desired degree. The optimum heat treatmentschedule depends, as will be understood, on the particular glasscomposition and its tendency to form nuclei, and the rate of formationof nuclei and the rate of crystallization. As previously indicated, itis thus not possible to specify a heat treatment schedule that will becommon to all the foregoing glasses.

However, it is usually preferred that the first-mentioned lowtemperature heat treatment be in a range of temperatures which promotesa high rate of formation of nuclei or crystallites, wherein nuclei" aredefined as sub-microscopic precursors of crystalline species or as afinely dispersed submicroscopic immiscible glassy phase. The mechanismof crystal initiation for the present glasses is not definitely known,nor is it known whether the first phase that separates during thecrystallization heat treatment schedule is an immiscible glassy phase oris a separate crystallite or crystalline phase. Also, it is difficult tomeasure directly the range of temperatures in which the higher rates ofnuclei formation occur, or in other words, where the optimum temperaturerange for the initial heattreatment is to be located. However, thistemperature range usually is from the annealing point of the glass to250 F. above the annealing point. The annealing point, as definedherein, can be determined by ASTM designations C 336-54T, with thetesting apparatus being calibrated using fibers of standard glasseshaving known annealing and strain points as specified and published bythe National Bureau of Standards.

While the best temperature range for maximum nuclei formation isdifficult to measure directly, the optimum initial low temperature heattreatment range can be empirically determined employing small dropletsof the glass and a micro-furnace capable of very rapid temperaturechange and accurate temperature control. A droplet of the glass, cooledto below the annealing point temperature, can be rapidly heated in themicro-furnace to a specific temperature between the annealing point and250 F. above the annealing point, and held at such temperature for aspecified time interval, the length of time of heating depending, again,upon the particular glass. Thus, if the glass inherently very rapidlyforms nuclei, a shorter standard time at the low temperature can be usedthan if the nuclei are relatively only slowly formed. In any event, asan example, a droplet of the glass can be heated for, say, 15 minutes at60 F. above the annealing point temperature. Thereafter the droplet ofglass in the micro-furnace can be very rapidly heated to a predeterminedcrystallization temperature, for instance, within the range l,750-l ,900F., and held at such predetermined temperature for a specific length oftime, for instance, one-half hour. This process can be repeated, usingthe same length of time of initial and final heating and the sametemperature of final heating, but using different initial heatingtemperatures, say 40 l00", and F. above the annealing point temperature.Thereafter by microscopic examination, one can determine which initialheat treatments resulted in formation of the most and smallest crystals,and thus determine the approximate temperature range where a maximumnumber of crystallization centers are formed. Thereafter, an optimumheat treatment schedule can be worked out by varying the length of timein the initial heat treatment range that appears to be optimum and byvarying time and temperatures of heating in thefinal crystallizationheat treatment range. Properties such as the fineness of the crystalsand the strength of samples treated according to various temperatureschedules can be determined as an aid in picking an optimum heattreatment schedule for the properties desired.

The process of thermal in situ crystallization thus usually comprisesheat treating the formed article in an optimum initial temperature rangebetween the annealing point and 250 F. above the annealing point for atime of at least one-half hour, usually at least 1 hour, and thereafterheat treating in a higher crystallization temperature range. The time ofinitial heat treatment in the range from the annealing point to 250 F.above the annealing point has no upper limit; usually it is not morethan 5 or 6 hours, but longer times are not in the least harmful andmerely increase the cost of processing. The crystallization heattreatment stage is effected at higher temperatures in the range fromabout l,700-l ,950 F with a sufficient length of time of heating in thehigh temperature range to effect in situ crystallization to at least theextent that the resulting glass-ceramic product after cooling to roomtemperature and reheating, will not substantially deform under its ownweight when held for one hour at a temperature 400 F. above theannealing point of the original glass. Thus, a rod, 7 inches long andabout one-fourth inch in diameter supported near each end by knife edgesspaced 6 inches apart will not deform or sag at the center as much asone-eighth inch. Obviously, a degree of crystallization that passes thistest represents a rather highly crystalline material, since glass orglass with only 5-10% crystalline material would obviously deform badlywhen held at a temperature so far above its annealing point. However, itis not possible to determine the exact relative amounts of crystallineand vitreous material in such densely crystallized materials as areproduced by the present invention. Generally, times of heating in thetemperature range of 1,700 to l,950 are from 15 minutes to 6 hours,usually from 1/2 to 4 hours.

In any event, the overall heat treatment chosen, that is, the initial ornucleation heat treatment and the crystallization heat treatment,efiected at the higher temperature, results in an at least partiallycrystalline ceramic body whose entire interior contains a multitude ofrandomly oriented, substantially homogeneously dispersed crystals,essentially all of which crystals are in their largest lineal dimensionless than 30 microns across. The products are densely crystallized,hard, and non-porous.

As will be understood, when going from the initial or nucleation heattreatment temperature to the higher crystallization temperature, it isusually preferred to proceed slowly enough or to stop at intermediateplateaus long enough, to effect appreciable crystallization in theintermediate temperature range, at least to such a degree that a rigidcrystalline network is formed that prevents the article from slumping.Of course, in heat treating articles such as flat plates that can becase in a mold and heat treated in the mold, the slumping problem is notimportant and not as much care need be exercised.

Although the specific examples show several plateaus of heat treatmenttemperatures, the entire heat treatment can be effected using slowly andcontinuously rising temperatures, and it is often desirable to employdifferent heating rates at various parts of the process. For instance,in the nucleation heat treatment temperature range the heating rate isusually slower than when going from this lower temperature range to thefinal crystallization temperature range.

As determined by X-ray powder diffraction measurements, the products ofthe crystallization step contain nepheline or a nepheline-likecrystalline phase as the major crystalline phase. In other words, thenepheline or nepheline-like crystalline phase is present in the ceramicproduct in much larger volume than any other crystalline phase, asdetermined by X-ray powder diffraction data.

The selected boron compound used for treating such glassceramics duringor after formation by crystallization must be boric oxide or bepyrolyzable (decomposable under the influence of heat) to boric oxide.In most instances, the choice of the compound utilized will be based onthe process economics.

The thickness of the vitreous, borosilicate compressive layer producedby the method of this invention depends in part on the length of time ofexposure to the boric oxide enriched atmosphere at the elevatedtemperatures. The time of exposure is then determined in advance basedon the expected service conditions of semi-crystalline articles undertreatment. If the semi-crystalline body is to be subjected to conditionsof severe abrasion and abuse, it is understood that a longer boroncompound exposure is desirable in order to obtain a relatively thickervitreous borosilicate layer.

The length of time of exposure to the boron enriched atmosphere willgenerally range from 10 minutes to about 5 hours. The maximumtemperature of the glass-ceramic body surface during such exposure ispreferably at least l,400 F. for the times indicated. Longer exposuretimes are not necessary but can be used. When the B exposure is effectedduring crystallization heat treatment, the time of contact of thecrystallizing body with B 0 vapors may be over 5 hours because thecrystallization heating cycle may be over such length oftime.

l have found that the normal crystallization heat treatment cyclecarried out in the presence of boric oxide vapor produces significantimprovements in the strength. Present information from analysis ofhydrofluoric acid etched layers shows that the surface borosilicatelayer formed during the crystallization process is usually more than 5and less than 100 microns in thickness. At any rate, I have found thateffective thicknesses can be accomplished without drastically departingfrom the normal crystallization heat cycle required for the alkalialuminosilicate systems.

At any rate tests show that the vitreous layer is integral with theglass-ceramic substrate. At the elevated temperatures specified, theboron oxide, whether it be derived from inorganic or organic material,reacts with a surface layer of the silicate substrate, forming a surfaceglassy layer, a mutual solution comprising boric oxide, aluminum oxide,alkali metal oxide and silica. It is emphasized that the reason it ispossible to achieve this in situ formation of a vitreous compressivelayer, and yet to avoid the development of surface cracks and fractures,is that the coefficient of expansion of the vitreous layer is in properrelationship to (i.e., lower than) the expansion coefficient of thesubstrate. The compressive stress develops in the vitreous borosilicatelayer on cooling the composite structure.

The glasses can be melted in the normal manner in gas-fired furnaces,preferably using slightly oxidizing conditions, or in electric furnacesfrom normal, common batch materials. Electric boosting can be providedin gas fired furnaces where desired. In the laboratory platinumcrucibles can be used. In-

larger furnaces high quality refractories are employed, such as highalumina refractories. When employing alumina refractories, it must beremembered that some alumina may enter the composition from therefractories, the amount depending in part upon the volume of charge inrelation to the surface area of the furnace, temperature, length of timeof melting, etc. Some adjustment in the batch composition may benecessary to account for the alumina from the refractory.

In a typical example of the method of the invention, flint sand, highpurity alumina, high purity rutile and GP. grade sodium carbonate weremelted to a glass in a platinum crucible in a gas-fired furnace usingslightly oxidizing conditions. Melting time was 22 hours at 2,800 F.,with mechanical stirring. Table I shows the composition and propertiesof the glass formed, and some properties of the glass-ceramic formed bythe heat treatment set forth in Example I.

TABLE I Oxide Composition and Properties of a Bulk crystallized whenheat treated as in Example I X l0"lC (IV-300C) The predominantcrystalline phase in the glass-ceramic was nepheline, when heat treatedas in Example I.

EXAMPLE I Glass rods were pulled from the composition of Table I byconventional forming techniques. Several samples of these rods were thensubjected to a crystallization cycle in a conventional electriclaboratory furnace, as follows:

Time (hours) Temperature (H 1 1,800 At the end of this treatment therods were gradually cooled to room temperature. The samples wereobserved to have crystallized. This example was designed to serve as acontrol so the benefit of the process of invention can be readilydetermined.

The modulus of rupture of the bulk crystallized samples was thendetermined by conventional techniques utilizing three point loading onTinius-Olsen equipment. In addition, the modulus of rupture wasdetermined for samples of the crystallized glass that had been subjectedto a laboratory abrasion cattechnique. This abrasion technique wasdeveloped to simulate very severe service, conditions and comprisesmounting the rod in a drill chuck and rotating the sample slowly for afew seconds while 320 grit abrasive paper is held in contact with thecenter of the sample.

The modulus of rupture data is reported in Table ll.

EXAMPLE II Several samples crystallized in Example I, were heated to1,800" F. in a standard laboratory electric furnace. When thetemperature was stabilized at 1,800" F., a small amount of comminutedC.P. grade boric anhydride (B was introduced into the closed furnace inan open crucible and allowed to vaporize. After 1 hour at 1,800 F., thesamples were allowed to cool slowly to room temperature. The sampleswere observed to be coated with a glossy, integral and stronglyadherent. vitreous borosilicate glaze layer which was under compressivestress. The modulus of rupture was determined according to the method ofExample I in both the abraded and unabraded conditions as previouslydescribed. The data is reported in Table ll.

EXAMPLE Ill Example I was duplicated in every detail except that a smallamount of comminuted C.P. grade boric anhydride was placed in the closedelectric furnace in an open crucible before the heat treatment wasbegun, and 8,0 vapors were maintained therein during the heat treatment.in this embodiment, conversion of the glass to the glass-ceramic wasaccomplished in the presence of vapors of B 0 At the end of thistreatment, the samples were similar in appearance to those reported inExample 11 and contained an integral, vitreous borosilicate layer morethan microns thick and under compressive stress. The modulus of rupturefor the abraded and unabraded samples are set forth in the followingTable II.

TABLE II Modulus of Rupture Data for Experiments Described in thePrededing Examples Modulus of Rupture (p.s.i.)

Example No. unabraded abraded l (control) 20,200 15,300

boron treatment of the surface with a member selected from 8,0; and aboron compound yielding B 0 under the conditions of treatment is attemperatures high enough 1,400 F. or higher) to form the borosilicateglassy layer.

Usually the boron compound starting material used is inorganic. Suitableinorganic boron compounds include boric oxide, boric acid, hydrated anddehydrated alkaline earth metal borates, hydrated and dehydrated alkalimetal borates and ammonium borate.

While the temperature required to form the vitreous layer is at least1,400 F it is known that the vitreous layer will form more rapidly atthe higher temperatures in the range of 1,800' 2,000 F. Practicalconsiderations discourage forming the vitreous layer at such hightemperatures that the semi-crystalline body substrate material begins todeform.

The strengthened glass-ceramic products of the present invention areuseful in many applications where conventional ceramics are used; forinstance, especially useful products are tableware-plates, cups, saucersand the like, where superior strength is a distinct asset.

While preferred embodiments have been described above in detail. it willbe understood that various modifications can be followed withoutdeparting from the spirit and scope of the disclosure or from the scopeof the following claims.

lclaim:

l. The method of making a strong, high expansion ceramic body resultingfrom thermal crystallization of an aluminosilicate glass body comprisingSiO,, A1 0 and a nucleant selected from TiO ZrO SnO, and Cr O present inthe glass composition in an amount from 0.1 to 12 weight percent, whichcomprises contacting said body with boric oxide vapors at a temperatureabove 1,400" F. for at least 10 minutes, the initial contacting of saidbody with said boric oxide being effected while said body is still inthe glassy state, said contacting being effected until said boric oxidevapors have reacted with surface layers of said body to form an integralsurface layer of borosilicate glass on said body, and continuing heatingsaid body and thereby thermally crystallizing said body to a glassceramic body, having said integral surface layer and having a linealcoefficient of thermal expansion of at least 50 X l0- C. over the range0 to 300 C. and cooling the resulting composite body, therby producing acompressive stress in said glass layer. 'A The product of the process ofclaim 1.

3. The method of making a strong, high expansion ceramic body resultingfrom thermal crystallization of an aluminosilicate glass body comprisingSiO A1 0 and a nucleant selected from TiO ZrO Sn0 and Cr O present inthe glass composition in an amount from 0.1 to 12 weight percent, whichcomprises contacting said body with boric oxide vapors after the glassbody is converted to a high expansion at least partially crystallineceramic body by thennal in situ crystallization of said aluminosilicateglass body, said contacting of said body with said boric oxide vaporstaking place at a temperature above 1,400 F. for at least 10 minutesuntil said boric oxide vapors have reacted with surface layers of saidceramic body to form an integral surface layer of borosilicate glass onsaid ceramic body, the lineal coefficient of thermal expansion of saidceramic body being at least 50 X l0"/C. over the range 0 to 300 C. andcooling the resulting composite body, thereby producing a compressivestress in said glass layer.

4. A product of the process of claim 3.

2. The product of the process of claim
 1. 3. The method of making astrong, high expansion ceramic body resulting from thermalcrystallization of an aluminosilicate glass body comprising SiO2, Al2O3and a nucleant selected from TiO2, ZrO2, SnO2 and Cr2O3 present in theglass composition in an amount from 0.1 to 12 weight percent, whichcomprises contacting said body with boric oxide vapors after the glassbody is converted to a high expansion at least partially crystallineceramic body by thermal in situ crystallization of said aluminosilicateglass body, said contacting of said body with said boric oxide vaporstaking place at a temperature above 1, 400* F. for at least 10 minutesuntil said boric oxide vapors have reacted with surface layers of saidceramic body to form an integral surface layer of borosilicate glass onsaid ceramic body, the lineal coefficient of thermal expansion of saidceramic body being at least 50 X 10 7/*C. over the range 0 to 300* C.and cooling the resulting composite body, thereby producing acompressive stress in said glass layer.
 4. A product of the process ofclaim 3.